In holographic recording, a light beam from a coherent monochromatic source (usually a laser) is split into two parts. One part of the beam (the object beam) is passed through a spatial light modulator (SLM) and then into a recording medium. A suitable SLM is a planar liquid crystal array having a plurality (e.g., 1024.times.1024) of individual liquid crystal panels. Each crystal panel has a transmissive state and a nontransmissive state. During the recording of a single two-dimensional image, some of the panels are in their transmissive (ON) state and some are in their nontransmissive (OFF) state. The other unmodulated portion of the beam strikes the recording medium directly (reference beam). The object beam and reference beam intersect in the recording medium to form a complex interference pattern which is recorded in the medium. After a data page has been recorded in the recording medium, the reference beam angle is changed and the next data page is recorded. In the reconstruction (readout) step, the reference beam alone strikes the recording medium and is diffracted by the recording medium to form a faithful copy of the original object beam. The diffracted beam is directed to a suitable detector, such as a charge-coupled device (CCD) detector array, which reads each pixel in the data page.
Unfortunately, during the readout process, errors may occur due to the lack of contrast between ON and OFF pixels. The lack of contrast is due to random noise and deterministic variations. Random noise is caused by camera noise, crosstalk from other holograms, and fidelity loss during recording.
Deterministic variations are the same from data page to data page and are caused by nonuniformities in the object beam before its modulation, dust and other imperfections in the optics, nonuniform pixel response in the SLM or CCD, interpixel crosstalk, and absorption effects in the photosensitive crystal. Deterministic variations cause some ON pixels in the reconstructed data pages to be repeatably weaker than other ON pixels. Further, defects or imperfections in the nontransmissive state of the SLM can cause some OFF pixels to record a weak signal. The lack of contrast between ON and OFF pixels can result in errors during readout.
An intuitive response to this problem would be to increase the recording time of each data page so that all ON pixels get brighter, thereby enhancing the contrast between ON and OFF pixels. Unfortunately, stored holograms are slowly erased during the recording process of a new hologram and increasing the recording time for each page merely exacerbates the erasure problem. Further, increasing recording time for each page also increases crosstalk from bright ON pixels into adjacent OFF pixels. There is still a need in the art for a recording method for enhancing the contrast between ON and OFF pixels.
It is therefore an object of the present invention to provide an improved method for optical data storage.
Other objects and advantages will become apparent from the following disclosure.